Impregnation of an hvof coating by a lubricant

ABSTRACT

The invention provides a method of protecting a metal substrate coated in a coating of WC—Co—Cr type by the HVOF method, the method comprising applying a lubricating agent on the coating, which lubricating agent comprises particles of solid lubricant incorporated in a fluid having surface tension that is low enough to penetrate into the pores of the coating and to entrain the solid lubricant particles therein.

The invention relates to a method of protecting a metal substrate coated by the HVOF method.

BACKGROUND OF THE INVENTION

For several years, and in particular in the field of aviation, coatings made by depositing hard chromium have been replaced progressively by deposits of a powder mixture of WC—Co—Cr (the powder mixture containing tungsten carbide, cobalt, and chromium) by using a high velocity oxy-fuel (HVOF) method. An HVOF method is a method of spraying that makes use of the combustion of a fuel and an oxidizer for the purpose of generating gas, which gas serves to eject the powder mixture onto a substrate at speeds and at temperatures that are high enough for the sprayed powder mixture to attach to the substrate and form a coating layer thereon.

In general, coating layers deposited in this way have the reputation of being very dense, and it is usual to improve the friction qualities of a mechanical part that is coated in this way by greasing it. By way of example, the grease is applied when inserting the part into the mechanism in which it is be installed and where it is liable to come into contact with another part during relative motion. Naturally, the WC—Co—Cr layer is machined beforehand (e.g. by grinding) in order to impart the design dimensions to the part as coated in this way.

Nevertheless, while the mechanism is in operation, the grease can disappear from the contact, leading to a sudden change of friction conditions between the two parts as a result of them rubbing bare against each other, thereby leading both to a drop in the sliding qualities of the parts and also to accelerated wear. The conventional solution is to perform greasing often enough to avoid the grease disappearing totally. Nevertheless, circumstances can exist (forgetting to perform greasing, utilization that is particularly intense, beyond that which was expected) that mean that the disappearance of the grease is not impossible.

OBJECT OF THE INVENTION

An object of the invention is to protect a metal substrate coated by the HVOF method in a coating, preferably a coating of WC—Co—Cr type, in order to avoid the risk of rubbing bare.

SUMMARY OF THE INVENTION

In order to achieve this object, the invention provides a method of protecting a metal substrate coated in a coating, preferably of WC—Co—Cr type, by the HVOF method, the method comprising applying a lubricating agent on the coating, which lubricating agent comprises particles of solid lubricant incorporated in a fluid having surface tension that is low enough to penetrate into the pores of the coating and to entrain the solid lubricant particles therein.

The inventors have observed that although coatings deposited by the HVOF method, and in particular coatings of the WC—Co—Cr type, are very dense, they nevertheless present a high level of interconnected porosity, which may exceed 15%, but with pores that are very small in size (typically of the order of 350 nanometers (nm)). The presence of such pores has passed unnoticed until now because pores that are so small are generally not detected by porosity measurement methods involving analyzing images at magnifications of ×200 to ×400, as are commonly used in industry. Nevertheless, such pores can be revealed by mercury porosimetry, which can also reveal a large amount of connectivity between pores.

Under such circumstances, by selecting a lubricating agent having fluid that presents surface tension that is low enough to obtain capillary action that is sufficient to infiltrate into the pores and the cracks in the coating, it is possible to incorporate solid lubricant into the thickness of the coating itself, which solid lubricant becomes spread throughout the thickness of the coating because of the very high level of connectivity between the pores.

A self-lubricating surface is thus created from which the solid lubricant is released and exerts its lubricating power progressively as the coating is worn away, without there being any need to re-lubricate.

Ideally, the lubricating agent containing particles of lubricant is applied in an environment at reduced pressure, in a vacuum, or at a pressure higher than atmospheric pressure. During application of the lubricating agent, it is also possible to vary pressure between low pressure, such as an airvoid, and a high pressure of the surroundings. This seeks to facilitate penetration of the lubricant into the pores in order to impregnate the deposited coatings correctly.

Ideally, after the lubricating agent has been applied and once the lubricant has penetrated into the array of interconnected open pores of the coating, an operation is performed of mechanically closing the pores of the coating, at least in part, by machining and/or grinding and/or polishing a surface portion of the coating. The mechanical operation of machining and/or grinding and/or polishing has the effect of plastically deforming at least some of the pores that are situated close to the surface of the coating (i.e. in a surface zone of the coating layer). This operation serves to close, at least in part, some of the pores situated at the surface of the coating, thereby limiting the passage for releasing the lubricant that has penetrated into the array of interconnected pores. This creates a coating that presents an array of interconnected pores containing lubricant with some of the pores in the proximity of the outer surface of the coating being closed and others being open. The lubricant is thus made accessible from the outside of the coating only via open pores that lead to the outside of the coating. As the coating becomes worn away progressively, the closed or partially closed pores open up and release a portion of the lubricant they contain. The coating thus presents an incorporated lubricating function that releases its lubricant at the same time as it becomes worn.

DETAILED DESCRIPTION OF THE INVENTION

By way of example, it is proposed to use the HVOF method for coating landing gear sliding rods that are made of high strength steel (300M steel in AMS standard 6257, or Aermet 100 steel in AMS standard 6532, after heat treatment in the range 1900 megapascals (MPa) to 2000 MPa) with a coating of WC-10Co-4Cr type having a mean size of agglomerates lying in the range 15 micrometers (μm) to 45 μm, which is obtained from standard elementary tungsten carbide (in the range 1 μm to 4 μm).

The selected lubricating agent is prepared from a hydrophobic fluid of WADIS24 type from the supplier Socomore Finishing Solutions. In the context of the invention, the fluid is enriched by incorporating therein particles of solid lubricant, e.g. of graphite, of molybdenum disulfide, or of tungsten disulfide. Naturally, care should be taken to ensure that the solid lubricant as incorporated in this way is of a size that is small enough to be capable of penetrating into the pores of the coating.

The lubricating agent is deposited on the coating by spraying or with a paint brush. The fluid forms a sealing monomolecular film at the surface of the coating and permeates into the pores of the coating, entraining the solid lubricant particles therewith.

A part is thus obtained that has a self-lubricating HVOF coating from which the solid lubricants are released progressively as the coating wears in service.

The recommended fluid is compatible with mineral oils, such that the part can be put rapidly into contact with another lubricant. Nevertheless, if that lubricant should run out during the lifetime of the part, the self-lubricating mechanism takes over in order to protect the part from any risk of rubbing bare.

Furthermore, the fluid has the effect of expelling water from the surface of the rod and it thus contributes to protecting the rod against corrosion.

Such treatment may be performed after the coating has been ground. Nevertheless, such treatment can also be performed before grinding, since that increases penetration of the lubricant into the preferably mutually interconnected open pores (where grinding has the effect of closing pores that are situated at the surface of the coating). Under such circumstances, the lubricant is released progressively as the coating wears, and in particular progressively during the wear of pores that are closed or partially closed and that contain the lubricant.

The invention is not limited to the above description, but on the contrary covers any variant coming within the ambit defined by the claims.

In particular, although the presently-described lubricating agent contains a WADIS24 hydrophobic fluid, it is naturally possible to use any other fluid having surface tension that is low enough to encourage penetration into the pores of the coating, e.g. perchloroethylene (tetrachloro ethylene). By way of example, it is possible to use micrometric molybdenum disulfide, and graphite mixed with an inorganic binder agent, e.g. sodium silicate, with that mixture being wetted with perchloroethylene. 

1. A method of protecting a metal substrate coated in a coating by the HVOF method, the method comprising applying a lubricating agent on the coating, which lubricating agent comprises particles of solid lubricant incorporated in a fluid having surface tension that is low enough to penetrate into the pores of the coating and to entrain the solid lubricant particles therein.
 2. The method according to claim 1, wherein the coating is of the WC—Co—Cr type.
 3. The method according to claim 1, wherein the solid lubricant particles are selected from graphite, molybdenum disulfide, and tungsten disulfide.
 4. The method according to claim 1, wherein, after applying the lubricating agent, a mechanical operation is performed of closing the pores of the coating, at least in part, by machining and/or grinding and/or polishing a surface portion of the coating. 